Recycling superabsorbent polymer fines

ABSTRACT

A process is described for recycling superabsorbent polymer fines into a process that includes treating the superabsorbent polymer fines with caustic and a polymerization step for making the superabsorbent polymer gel. The process requires treating the superabsorbent polymer fines with a caustic, followed by mixture with polymerizable monomer solution, and polymerizing the mixture of the superabsorbent polymer fines and monomer to form the aqueous fluid absorbent polymer. In the process, the fines are incorporated into the new polymer gel and become indistinguishable there from. The gel may then be comminuted into a particulate dried and then separated into a portion having a desired minimum particle size in a fines portion having less than the desired size. The particulate may then be coated with a surface crosslinking agent and surface additives and heated for surface conversion.

This application is a divisional application of U.S. application Ser. No. 13/020,898 filed on Feb. 4, 2011, which is a continuation application of U.S. application Ser. No. 12/256,038 filed on Oct. 22, 2008, now U.S. Pat. No. 7,910,688, the disclosures of which are expressly incorporated herein by reference.

BACKGROUND

A superabsorbent material in general refers to a water-swellable, water-insoluble, material capable of absorbing at least about 10 times its weight, and up to about 30 times or more its weight in an aqueous solution containing 0.9 weight percent sodium chloride solution in water. The present invention relates to superabsorbent polymer particles, which absorb water, aqueous liquids, and blood, and a method to make the superabsorbent polymer and particles. The acronym SAP may be used herein, and is generally used in the industry, in place of superabsorbent polymer, superabsorbent polymer composition, superabsorbent polymer particles, or variations thereof.

A superabsorbent polymer is a cross linked partially neutralized polymer that is capable of absorbing large amounts of aqueous liquids and body fluids, such as urine or blood, with swelling and the formation of hydrogels, and of retaining them under a certain pressure in accordance with the general definition of superabsorbent material. A superabsorbent polymer composition is a superabsorbent polymer that has been surface treated that may include surface cross linking, surface treatment, and other treatment. Commercially available superabsorbent polymer compositions include cross linked polyacrylic acids or cross linked starch-acrylic acid graft polymers, in which some of the carboxyl groups are neutralized with sodium hydroxide solution or potassium hydroxide solution. Superabsorbent polymer composition particles are particles of superabsorbent polymers or superabsorbent polymer compositions, and generally have a particle size of from about 150 microns to about 850 microns. A comprehensive survey of superabsorbent polymers, and their use and manufacture, is given in F. L. Buchholz and A. T. Graham (editors) in “Modern Superabsorbent Polymer Technology,” Wiley-VCH, New York, 1998.

A primary use of SAP and SAP particles is in sanitary articles, such as babies' diapers, incontinence products, or sanitary towels. For fit, comfort, and aesthetic reasons, and from environmental aspects, there is an increasing trend to make sanitary articles smaller and thinner. This is being accomplished by reducing the content of the high volume fluff fiber in these articles. To ensure a constant total retention capacity of body fluids in the sanitary articles, more SAP content is being used in these sanitary articles.

The present invention relates generally to superabsorbent polymer composition particles having a particle size of about 150 microns or less, which are generally known to those skilled in the art and referred herein as superabsorbent polymer fines, SAP fines, or fines. The present invention relates to recycling the superabsorbent polymer fines during the production process. More particularly, the invention relates to reprocessing superabsorbent polymer fines generated in the production of superabsorbent polymer particles.

In particular, SAP fines are generally generated during the cutting, comminution (pulverization), and grinding steps of the dried polymer gel. Superabsorbent polymer fines are considered to be undesirable in sanitary articles including infant diapers and adult incontinence devices. Such SAP fines may migrate in the device before use and exhibit gel blocking in application. In addition, it has been found that superabsorbent polymer particles made including a portion of recycled superabsorbent polymer fines have properties including absorption as measured by the Centrifuge Retention Capacity test (CRC), absorbency under load (AUL), and permeability (GBP) that are less desirous than the superabsorbent polymer particles that are made from a process that does not include recycling superabsorbent polymer fines.

U.S. Pat. No. 5,514,754 is directed to a process for increasing the size of fine particles of SAP (cross-linked polyacrylic acid, cross-linked polyacrylate, and/or cross-linked partially neutralized polyacrylic acid) and, more particularly, to a process of impregnating the SAP particles with an aqueous monomer solution of acrylic acid and/or metal salts of acrylic acid. Impregnation is achieved by intimately mixing the SAP fines (e.g., having a particle size less than about 300 μm) with the monomer solution to form a paste having a viscosity of, e.g., about 20,000 to about 5,000,000 centipoise. Polymerization of a monomer within, and drying of, the paste then is achieved, e.g., by heat.

It is an object of the present invention to provide superabsorbent polymer particle compositions made by a process that includes recycled superabsorbent polymer fines wherein the superabsorbent polymer particle compositions have improved properties including an absorption capacity to retain fluids under no load, high absorption capacities to retain fluid under pressure as measured by absorption under load at 0.9 psi, and improved gel bed permeability.

SUMMARY

An embodiment of the present invention comprises a process for the production of a superabsorbent polymer gel with recycled superabsorbent polymer fines comprising the following steps: a) treating the superabsorbent polymer fines with a caustic solution containing from about 0.1 to about 12% caustic based on the weight of the superabsorbent polymer fines; b) mixing the treated superabsorbent polymer fines obtained in step a) with a polymerization solution containing at least one cross-linking-agent and a partially neutralized monomer in which the content of the superabsorbent polymer fines relative to the total amount of the monomer is from about 0.1 to about 30 wt %; and c) polymerizing the mixture obtained in step b) to produce the superabsorbent polymer gel.

In addition the process may further comprise the steps of comminuting the superabsorbent polymer gel from said polymerizing step; drying said comminuted polymer gel; separating said dried superabsorbent polymer particle composition into a portion having a desired particle size from about 150 μm to about 850 μm.

The process may further comprise the steps to make a superabsorbent polymer composition wherein the steps comprise coating the superabsorbent polymer particulate with a surface cross linking agent and surface additives, heating the coated superabsorbent polymer composition, and separating said dried superabsorbent polymer particulate into a portion having a desired particle size from about 150 μm to about 850 μm as measured by screening through a U.S. standard 20 mesh screen and retained on a U.S. standard 100 mesh screen.

And furthermore, the process may further comprise the steps of separating superabsorbent polymer fines having a particle size of less than about 150 μm and recycling said superabsorbent polymer fines to the polymerizing step for forming said superabsorbent polymer composition particle.

An embodiment of the present invention further includes a superabsorbent polymer particulate made by the foregoing process. In addition, the present invention is directed to absorbent compositions or sanitary articles such as diapers that may contain superabsorbent polymer compositions of the present invention.

It has been found that the caustic treatment on superabsorbent polymer fines increases certain properties of the superabsorbent polymer particle composition made by a process that incorporates caustic treated superabsorbent polymer fines.

Numerous other features and advantages of the present invention will appear from the following description. In the description, reference is made to exemplary embodiments of the invention. Such embodiments do not represent the full scope of the invention. Reference should therefore be made to the claims herein for interpreting the full scope of the invention. In the interest of brevity and conciseness, any ranges of values set forth in this specification contemplate all values within the range and are to be construed as support for claims reciting any sub-ranges having endpoints which are real number values within the specified range in question. By way of a hypothetical illustrative example, a disclosure in this specification of a range of from 1 to 5 shall be considered to support claims to any of the following ranges: 1-5; 1-4; 1-3; 1-2; 2-5; 2-4; 2-3; 3-5; 3-4; and 4-5.

FIGURES

The foregoing and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:

FIG. 1 is a side view of the test apparatus employed for the Free Swell Gel Bed Permeability Test;

FIG. 2 is a cross-sectional side view of a cylinder/cup assembly employed in the Free Swell Gel Bed Permeability Test apparatus shown in FIG. 1;

FIG. 3 is a top view of a plunger employed in the Free Swell Gel Bed Permeability Test apparatus shown in FIG. 1; and

FIG. 4 is a side view of the test apparatus employed for the Absorbency Under Load Test;

DEFINITIONS

It should be noted that, when employed in the present disclosure, the terms “comprises,” “comprising,” and other derivatives from the root term “comprise” are intended to be open-ended terms that specify the presence of any stated features, elements, integers, steps, or components, and are not intended to preclude the presence or addition of one or more other features, elements, integers, steps, components, or groups thereof.

The term “absorbent article” generally refers to devices that can absorb and contain fluids. For example, personal care absorbent articles refer to devices that are placed against or near the skin to absorb and contain the various fluids discharged from the body.

The term “caustic” or “caustic soda” is used herein to describe a chemical that forms an alkaline solution when dissolved in a solvent such as water, including for example sodium hydroxide, sodium carbonate, or sodium bicarbonate.

The term “cross linked” used in reference to the superabsorbent polymer refers to any means for effectively rendering normally water-soluble materials substantially water-insoluble but swellable. Such a cross linking means can include, for example, physical entanglement, crystalline domains, covalent bonds, ionic complexes and associations, hydrophilic associations such as hydrogen bonding, hydrophobic associations, or Van der Waals forces.

The term “Darcy” is a CGS unit of permeability. One Darcy is the permeability of a solid through which one cubic centimeter of fluid, having a viscosity of one centipoise, will flow in one second through a section one centimeter thick and one square centimeter in cross-section, if the pressure difference between the two sides of the solid is one atmosphere. It turns out that permeability has the same units as area; since there is no SI unit of permeability, square meters are used. One Darcy is equal to about 0.98692×10⁻¹² m² or about 0.98692×10⁻⁸ cm².

The term “disposable” is used herein to describe absorbent articles that are not intended to be laundered or otherwise restored or reused as an absorbent article after a single use. Examples of such disposable absorbent articles include, but are not limited to, personal care absorbent articles, health/medical absorbent articles, and household/industrial absorbent articles.

The term “dry superabsorbent polymer composition” generally refers to the superabsorbent polymer composition having less than about 10% moisture.

The term “mass median particle size” of a given sample of particles of superabsorbent polymer composition is defined as the particle size, which divides the sample in half on a mass basis, i.e., half of the sample by weight has a particle size greater than the mass median particle size, and half of the sample by mass has a particle size less than the mass median particle size. Thus, for example, the mass median particle size of a sample of superabsorbent polymer composition particles is 2 microns if one-half of the sample by weight is measured as more than 2 microns.

The terms “particle,” “particulate,” and the like, when used with the term “superabsorbent polymer,” refer to the form of discrete units. The units can comprise flakes, fibers, agglomerates, granules, powders, spheres, pulverized materials, or the like, as well as combinations thereof. The particles can have any desired shape: for example, cubic, rod-like, polyhedral, spherical or semi-spherical, rounded or semi-rounded, angular, irregular, et cetera. Shapes having a high aspect ratio, like needles, flakes, and fibers, are also contemplated for inclusion herein. The terms “particle” or “particulate” may also include an agglomeration comprising more than one individual particle, particulate, or the like. Additionally, a particle, particulate, or any desired agglomeration thereof may be composed of more than one type of material.

The term “polymer” includes, but is not limited to, homopolymers, copolymers, for example, block, graft, random, and alternating copolymers, terpolymers, etc., and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term “polymer” shall include all possible configurational isomers of the material. These configurations include, but are not limited to isotactic, syndiotactic, and atactic symmetries.

The term “polyolefin” as used herein generally includes, but is not limited to, materials such as polyethylene, polypropylene, polyisobutylene, polystyrene, ethylene vinyl acetate copolymer, and the like, the homopolymers, copolymers, terpolymers, etc., thereof, and blends and modifications thereof. The term “polyolefin” shall include all possible structures thereof, which include, but are not limited to, isotatic, synodiotactic, and random symmetries. Copolymers include atactic and block copolymers.

The term “superabsorbent materials” refers to water-swellable, water-insoluble organic or inorganic materials including superabsorbent polymers and superabsorbent polymer compositions capable, under the most favorable conditions, of absorbing at least about 10 times their weight, or at least about 15 times their weight, or at least about 25 times their weight in an aqueous solution containing 0.9 weight percent sodium chloride.

The term “superabsorbent polymer composition” refers to a superabsorbent polymer comprising a surface additive in accordance with the present invention.

The terms “superabsorbent polymer” and “superabsorbent polymer preproduct” refer to a material that is produced by conducting all of the steps for making a superabsorbent polymer as described herein, up to and including drying the material, and coarse grinding in a crusher.

The term “superabsorbent polymer fines” refers to a fraction of fine particles produced in the production process of superabsorbent polymers, having particles less than a certain size that may be about 300 microns, or about 200 microns, or about 150 microns.

The term “surface cross linking” means that the level of functional cross links in the vicinity of the surface of the superabsorbent polymer particle generally is higher than the level of functional cross links in the interior of the superabsorbent polymer particle. As used herein, “surface” describes the outer-facing boundaries of the particle. For porous superabsorbent polymer particles, exposed internal surface also are included in the definition of surface.

The term “thermoplastic” describes a material that softens when exposed to heat and which substantially returns to a non-softened condition when cooled to room temperature.

The term “% by weight” or “% wt” when used herein and referring to components of the superabsorbent polymer composition, is to be interpreted as based on the weight of the dry superabsorbent polymer composition, unless otherwise specified herein.

These terms may be defined with additional language in the remaining portions of the specification.

DETAILED DESCRIPTION

An embodiment of the present invention comprises a process for the production of a superabsorbent polymer gel with recycled superabsorbent polymer fines comprising the following steps: a) treating the superabsorbent polymer fines with a caustic solution containing from about 0.1 to about 12% caustic based on the weight of the superabsorbent polymer fines; b) mixing the treated superabsorbent polymer fines obtained in step a) with a polymerization solution containing at least one cross-linking-agent and a partially neutralized monomer in which the content of the superabsorbent polymer fines relative to the total amount of the monomer is from about 0.1 to about 30 wt %; and c) polymerizing the mixture obtained in step b) to produce the superabsorbent polymer gel.

In an embodiment of the present invention the superabsorbent polymer fines having an average particle size of less than about 300 μm, or about 200 μm, or about 150 μm.

In addition, the process may further comprise the steps of separating the superabsorbent polymer fines from the superabsorbent polymer composition particles, treating the superabsorbent polymer fines with caustic such as sodium hydroxide, or sodium carbonate, or sodium bicarbonate, and recycling the caustic treated superabsorbent polymer fines to the polymerizing step for forming said superabsorbent polymer composition particles.

In another embodiment, the present invention is directed to a process for the production of a superabsorbent polymer gel based on recycling superabsorbent polymer fines comprising the following steps: a) treating the superabsorbent polymer fines with a caustic solution containing from about 0.1 to about 12% caustic based on the weight of the superabsorbent polymer fines; b) mixing the treated superabsorbent polymer fines obtained in step a) with a polymerization solution containing at least one cross-linking-agent and a monomer in which the content of the superabsorbent polymer fines relative to the total amount of the monomer is from about 0.1 to about 30 wt %; c) polymerizing the mixture obtained in step b) to produce the polymer gel; and d) postneutralizing the polymer gel to form a superabsorbent polymer gel.

In another embodiment, the present invention is directed to a process for recycling superabsorbent polymer fines in the production of superabsorbent polymer gel, compromising the steps of: a) treating the superabsorbent polymer fines with a caustic solution containing from about 0.1 to about 12% caustic based on the weight of the superabsorbent polymer fines; b) mixing the treated superabsorbent polymer fines obtained in step a) with a polymerization solution containing at least one cross-linking-agent and a partially neutralized monomer in which the content of the superabsorbent polymer fines relative to the total amount of the monomer is from about 0.1 to about 30 wt %; and c) polymerizing the mixture obtained in step b) to produce a superabsorbent polymer gel.

In another embodiment, the present invention is directed to a process for recycling for recycling superabsorbent polymer fines in the production of superabsorbent polymer gel, compromising the steps of: a) dissolving at least one acrylic acid monomer and/or its sodium salt and a cross-linking agent in an aqueous solution to form a monomer solution; b) polymerizing the mixture obtained in step a) to produce a superabsorbent polymer gel. c) treating the superabsorbent polymer fines with a caustic solution containing from about 0.1 to about 12% caustic based on the weight of the superabsorbent polymer fines; and d) mixing the treated superabsorbent polymer fines obtained in step c) with the superabsorbent polymer gel obtained in step b) in which the content of the superabsorbent polymer fines relative to the total amount of the superabsorbent polymer gel is from about 0.1 to about 30 wt %.

A superabsorbent polymer as set forth in embodiments of the present invention is obtained by the initial polymerization of from about 55% to about 99.9% by weight of the superabsorbent polymer of polymerizable unsaturated acid group containing monomer. A suitable monomer includes any of those containing carboxyl groups, such as acrylic acid, methacrylic acid, or 2-acrylamido-2-methylpropanesulfonic acid, or mixtures thereof. It is desirable for at least about 50% by weight, and more desirable for at least about 75% by weight of the acid groups to be carboxyl groups.

The acid groups are neutralized to the extent of at least about 25 mol %, that is, the acid groups are desirably present as sodium, potassium, or ammonium salts. In some aspects, the degree of neutralization may be at least about 50 mol %. In some aspects, it is desirable to utilize polymers obtained by polymerization of acrylic acid or methacrylic acid, the carboxyl groups of which are neutralized to the extent of from about 50 mol % to about 80 mol %, in the presence of internal cross linking agents.

In some aspects, the suitable monomer that can be copolymerized with the ethylenically unsaturated monomer may include, but is not limited to acrylamide, methacrylamide, hydroxyethyl acrylate, dimethylaminoalkyl (meth)-acrylate, ethoxylated (meth)-acrylates, dimethylaminopropylacrylamide, or acrylamidopropyltrimethylammonium chloride. Such monomer may be present in a range of from 0% to about 40% by weight of the copolymerized monomer.

The superabsorbent polymer of the invention also includes internal cross linking agents. The internal cross linking agent has at least two ethylenically unsaturated double bonds, or one ethylenically unsaturated double bond and one functional group that is reactive toward acid groups of the polymerizable unsaturated acid group containing monomer, or several functional groups that are reactive towards acid groups can be used as the internal cross linking component and is desirably present during the polymerization of the polymerizable unsaturated acid group containing a monomer.

Examples of internal cross linking agents include, but are not limited to, aliphatic unsaturated amides, such as methylenebisacryl- or -methacrylamide or ethylenebisacrylamide; aliphatic esters of polyols or alkoxylated polyols with ethylenically unsaturated acids, such as di(meth)acrylates or tri(meth)acrylates of butanediol or ethylene glycol, polyglycols or trimethylolpropane; di- and triacrylate esters of trimethylolpropane which may be oxyalkylated, desirably ethoxylated, with about 1 to about 30 moles of alkylene oxide; acrylate and methacrylate esters of glycerol and pentaerythritol and of glycerol and pentaerythritol oxyethylated with desirably about 1 to about 30 mol of ethylene oxide; allyl compounds, such as allyl (meth)acrylate, alkoxylated allyl (meth)acrylate reacted with desirably about 1 to about 30 mol of ethylene oxide, triallyl cyanurate, triallyl isocyanurate, maleic acid diallyl ester, poly-allyl esters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, diols, polyols, hydroxy allyl or acrylate compounds and allyl esters of phosphoric acid or phosphorous acid; and monomers that are capable of cross linking, such as N-methylol compounds of unsaturated amides, such as of methacrylamide or acrylamide, and the ethers derived therefrom. Ionic cross linkers such as multivalent metal salts may also be employed. Mixtures of the cross linking agents mentioned can also be employed. The content of the internal cross linking agents is from about 0.001% to about 5% by weight such as from about 0.2% to about 3% by weight based on the total amount of the polymerizable unsaturated acid group containing monomer.

In some aspects, initiators can be used for initiation of the free-radical polymerization. Suitable initiators include, but are not limited to, azo or peroxo compounds, redox systems or UV initiators, sensitizers, and/or radiation.

After polymerization, the superabsorbent polymer is generally formed into particles. The superabsorbent polymer particles may then be surface cross linked after polymerization by the addition of a surface cross linking agent and heat-treatment. In general, surface cross linking is a process that is believed to increase the cross link density of the polymer matrix in the vicinity of the superabsorbent polymer particle surface with respect to the cross linking density of the particle interior.

In some particular aspects, desirable surface cross linking agents include chemicals with one or more functional groups that are reactive toward pendant groups of the polymer chains, typically the acid groups. The surface cross linking agent may be present in an amount of from about 0.001% to about 5% by weight of the dry superabsorbent polymer composition, and such as from about 0.1% to about 3% by weight, and such as from about 0.1% to about 1% by weight, based on the weight of the dry superabsorbent polymer composition. Applicants have found that a heat treatment step after addition of the surface cross linking agent is desirable.

In one particular aspect, the particulate superabsorbent polymer is coated or surface-treated with an alkylene carbonate followed by heating to affect surface cross linking, which can improve the surface cross linking density and the gel strength characteristics of the superabsorbent polymer composition particle. More specifically, the surface cross linking agent is coated onto the superabsorbent polymer particulate by mixing the polymer particulate with an aqueous alcoholic solution of the alkylene carbonate surface cross linking agent. The amount of alcohol is determined by the solubility of the alkylene carbonate and is kept as low as possible for various reasons. Suitable alcohols may include methanol, isopropanol, ethanol, butanol, or butyl glycol, as well as mixtures of these alcohols. In some aspects, the solvent desirably is water, which typically is used in an amount of about 0.3% by weight to about 5.0% by weight, based on the weight of the dry superabsorbent polymer composition. In other aspects, the alkylene carbonate surface cross linking agent is dissolved in water without any alcohol. In still other aspects, the alkylene carbonate surface cross linking agent may be applied from a powder mixture, for example, with an inorganic carrier material, such as silicone dioxide (SiO₂), or in a vapor state by sublimation of the alkylene carbonate.

To achieve the desired surface cross linking properties, the alkylene carbonate is distributed evenly on the particulate superabsorbent polymer. For this purpose, mixing is effected in suitable mixers known in the art, such as fluidized bed mixers, paddle mixers, rotary drum mixers, or twin-worm mixers. It is also possible to carry out the coating of the particulate superabsorbent polymer during one of the process steps in the production of the particulate superabsorbent polymer. In one particular aspect, a suitable process for this purpose is the inverse suspension polymerization process.

The heat treatment, that may follow the coating treatment, may be carried out as follows. In general, the heat treatment is at a temperature of from about 100° C. to about 300° C. Lower temperatures are possible if highly reactive epoxide cross linking agents are used. However, if alkylene carbonates are used, then the thermal treatment is suitably at a temperature of from about 150° C. to about 250° C. In this particular aspect, the treatment temperature depends on the dwell time and the kind of alkylene carbonate. For example, at a temperature of about 150° C., the thermal treatment is carried out for one hour or longer. In contrast, at a temperature of about 250° C., a few minutes (e.g., from about 0.5 minutes to about 5 minutes) are sufficient to achieve the desired surface cross-linking properties. The thermal treatment may be carried out in conventional dryers or ovens known in the art.

In some aspects, the superabsorbent polymer composition of the present invention may include from 0% to about 5% by weight, or from about 0.001% to about 5% by weight, or from about 0.01% to about 0.5% by weight of the dry superabsorbent polymer composition of a polymeric coating, such as a thermoplastic coating, or a cationic coating, or a combination of a thermoplastic coating and a cationic coating. In some particular aspects, the polymeric coating desirably is a polymer that may be in a solid, emulsion, suspension, colloidal, or solubilized state, or combinations thereof. Polymeric coatings suitable for this invention may include, but are not limited to, a thermoplastic coating having a thermoplastic melt temperature wherein the polymeric coating is applied to the particle surface coincident with or followed by a temperature of the treated superabsorbent polymer particle at about the thermoplastic melt temperature.

Examples of thermoplastic polymers include, but are not limited to, polyolefin, polyethylene, polyester, polyamide, polyurethane, styrene polybutadiene, linear low density polyethylene (LLDPE), ethylene acrylic acid copolymer (EAA), ethylene alkyl methacrylate copolymer (EMA), polypropylene (PP), maleated polypropylene, ethylene vinyl acetate copolymer (EVA), polyester, polyamide, and blends of all families of polyolefins, such as blends of PP, EVA, EMA, EEA, EBA, HDPE, MDPE, LDPE, LLDPE, and/or VLDPE, may also be advantageously employed. The term polyolefin as used herein is defined above. In particular aspects, the Applicants have found maleated polypropylene to be a desirable thermoplastic polymer for use in the present invention. A thermoplastic polymer may be functionalized to have additional benefits such as water solubility or dispersability.

Polymeric coatings of this invention may also include a cationic polymer. A cationic polymer as used herein refers to a polymer or mixture of polymers comprising a functional group or groups having a potential of becoming positively charged ions upon ionization in an aqueous solution. Suitable functional groups for a cationic polymer include, but are not limited to, primary, secondary, or tertiary amino groups, imino groups, imido groups, amido groups, and quaternary ammonium groups. Examples of synthetic cationic polymers include, but are not limited to, the salts or partial salts of poly(vinyl amines), poly(allylamines), poly(ethylene imine), poly(amino propanol vinyl ethers), poly(acrylamidopropyl trimethyl ammonium chloride), poly(diallyldimethyl ammonium chloride). Poly(vinyl amines) include, but are not limited to, LUPAMIN® 9095 available from BASF Corporation, Mount Olive, N.J. Examples of natural-based cationic polymers include, but are not limited to, partially deacetylated chitin, chitosan, and chitosan salts. Synthetic polypeptides such as polyasparagins, polylysines, polyglutamines, and polyarginines are also suitable cationic polymers.

The absorbent polymers according to the invention can comprise include from 0 to about 5 wt % of a multivalent metal salt, based on the weight of the mixture, on the surface of the polymer. The multivalent metal salt is preferably water soluble. Examples of preferred metal cations include the cations of Al, Fe, Zr, Mg and Zn. Preferably, the metal cation has a valence of at least +3, with Al being most preferred. Examples of preferred anions in the multivalent metal salt include halides, chlorohydrates, sulfates, nitrates and acetates, with chlorides, sulfates, chlorohydrates and acetates being preferred, chlorohydrates and sulfates being more preferred and sulfates being the most preferred. Aluminum sulfate is the most preferred multivalent metal salt and is readily commercially available. The preferred form of aluminum sulfate is hydrated aluminum sulfate, preferably aluminum sulfate having from 12 to 14 waters of hydration. Mixtures of multivalent metal salts can be employed.

The polymer and multivalent metal salt suitably are mixed by dry blending, or preferably in solution, using means well known to those skilled in the art. Aqueous solutions are preferred. With dry blending, a binder may be employed in an amount which is sufficient to ensure that a substantially uniform mixture of the salt and the superabsorbent polymer is maintained. The binder may be water or a nonvolatile organic compound having a boiling point of at least 150° C. Examples of binders include water, polyols such as propylene glycol, glycerin and poly(ethylene glycol).

The superabsorbent polymer compositions according to the invention may include from about 0.01% to about 2% by weight, or from about 0.01% to about 1% by weight based on the dry superabsorbent polymer composition of a water-insoluble inorganic metal compound. The water-insoluble inorganic metal compounds may include, but are not limited to, a cation selected from aluminum, titanium, calcium, or iron and an anion selected from phosphate, borate, or sulfate. Examples of water-insoluble inorganic metal compounds include aluminum phosphate and an insoluble metal borate. The inorganic metal compound may have a mass median particle size of less than about 2 μm, and may have a mass median particle size of less than about 1 μm.

The inorganic metal compound can be applied in the dry physical form to the surface of the superabsorbent polymer particles. For this, the superabsorbent polymer particles can be intimately mixed with the finely divided inorganic metal compound. The finely divided inorganic metal compound is usually added at about room temperature to the superabsorbent polymer particles and mixed in until a homogeneous mixture is present. For this purpose, mixing is effected in suitable mixers known in the art, such as fluidized bed mixers, paddle mixers, rotary drum mixers, or twin-worm mixers. The mixing of the SAP particles with the finely divided water-insoluble inorganic metal compound may take place before or after any surface cross linking, for example during the application of the surface cross linking agent.

Alternatively, a suspension of a finely divided water-insoluble inorganic metal compounds can be prepared and applied to a particulate SAP. The suspension is applied, for example, by spraying. Useful dispersion media for preparing the suspension include water, organic solvents such as alcohols, for example methanol, ethanol, isopropanol, ketones, for example acetone, methyl ethyl ketone, or mixtures of water with the aforementioned organic solvents. Other useful dispersion media include dispersion aids, surfactants, protective colloidals, viscosity modifiers, and other auxiliaries to assist in the preparation of the suspension. The suspension can be applied in conventional reaction mixers or mixing and drying systems as described above at a temperature in the range from room temperature to less than the boiling point of the dispersion medium, preferably at about room temperature. It is appropriate to combine the application of the suspension with a surface cross linking step by dispersing the finely divided water-insoluble metal salt in the solution of the surface cross linking agent. Alternatively, the suspension can also be applied before or after the surface cross linking step. The application of the slurry may be followed by a drying step.

In some aspects, the superabsorbent polymer compositions according to the invention can include from 0% to about 5%, or from about 0.01% to about 3%, by weight of the dry superabsorbent polymer composition of silica. Examples of silica include fumed silica, precipitated silica, silicon dioxide, silicic acid, and silicates. In some particular aspects, microscopic noncrystalline silicon dioxide is desirable. Products include SIPERNAT® 22S and AEROSIL® 200 available from Evonik Corporation, Parsippany, N.J. In some aspects, the particle diameter of the inorganic powder can be 1,000 μm or smaller, such as 100 μm or smaller.

In some aspects, the superabsorbent polymer compositions may also include from 0% to about 30% by weight of the dry superabsorbent polymer composition, such as from about 0.1% to about 5% by weight, of water-soluble polymers based by weight of the dry superabsorbent polymer composition, of partly or completely hydrolyzed polyvinyl acetate, polyvinylpyrrolidone, starch or starch derivatives, polyglycols, polyethylene oxides, polypropylene oxides, or polyacrylic acids.

In some aspects, additional surface additives may optionally be employed with the superabsorbent polymer particles, such as odor-binding substances, such as cyclodextrins, zeolites, inorganic or organic salts, and similar materials; anti-caking additives, flow modification agents, surfactants, viscosity modifiers, and the like. In addition, surface additives may be employed that perform several roles during surface modifications. For example, a single additive may be a surfactant, viscosity modifier, and may react to cross link polymer chains.

In some aspects, the superabsorbent polymer compositions of the present invention may be, after a heat treatment step, treated with water so that the superabsorbent polymer composition has water content of up to about 10% by weight of the superabsorbent polymer composition. This water may be added with one or more of the surface additives from above added to the superabsorbent polymer.

The superabsorbent polymer compositions according to the invention may be prepared either continuously or discontinuously in a large-scale industrial manner, the after-cross linking according to the invention being carried out accordingly. The partially neutralized monomer, such as acrylic acid, is converted into a gel by free-radical polymerization in aqueous solution in the presence of cross linking agents and any further components, and the gel is comminuted, dried, ground, and sieved off to the desired particle size. The superabsorbent polymer composition particles of the present invention generally include particle sizes ranging from about 150 to about 850 microns. The present invention may include at least about 40 wt % of the particles having a particle size from about 300 μm to about 600 μm, or at least about 50 wt % of the particles having a particle size from about 300 μm to about 600 μm, or at least about 60 wt % of the particles having a particle size from about 300 μm to about 600 μm as measured by screening through a U.S. standard 30 mesh screen and retained on a U.S. standard 50 mesh screen. In addition, the size distribution of the SAP particles of the present invention may include less than about 30% by weight of SAP particles having a size greater than about 600 microns, and less than about 30% by weight of SAP particles having a size of less than about 300 microns as measured using for example a RO-TAP® Mechanical Sieve Shaker Model B available from W. S. Tyler, Inc., Mentor Ohio.

It is well known to those skilled in the art that particle size distribution of the SAP particles resembles a normal distribution or a bell shaped curve. It is also known that for various reasons, the normal distribution of the particle size distribution may be skewed in either direction.

The SAP pre-product as used herein is produced by repeating all of the steps for making the SAP particles, up to and including drying the material, and coarse grinding the superabsorbent polymer in a crusher, and removing superabsorbent polymer particles greater than about 850 microns and smaller than about 150 microns. The particles smaller than about 300 μm, or about 200 μm, or about 150 μm are treated with a caustic solution and recycled in the polymerization process. The purpose of the caustic treatment is to partially hydrolyze the fines. The treated fines can be in slurry, paste, aggregate, or particulate forms.

In particular, treating the superabsorbent polymer fines with caustic may include a) treating the superabsorbent polymer fines with a caustic solution containing from about 0.1 to about 12 wt % caustic based on the weight of the fines aqueous solution. This can be done by mixing a caustic solution with superabsorbent polymer fines. For this purpose, mixing is effected in suitable mixers known in the art, such as fluidized bed mixers, paddle mixers, rotary drum mixers, or twin-worm mixers. The caustic solution is distributed evenly on the superabsorbent polymer fines, for example, by spraying or dripping.

The treated superabsorbent polymer fines obtained in step a) is mixed with a polymerization solution containing a cross-linking-agent and a neutralized monomer in which the content of the superabsorbent polymer fines relative to the total amount of monomers, amounts to 0.1 to 30 wt % and the mixture is polymerized to produce a superabsorbent polymer gel. This process may further include the steps of comminuting the aqueous fluid absorbent polymer gel from said polymerizing step; drying communicated polymer gel; separating the dried superabsorbent polymer particle composition into a portion having a desired particle size from about 150 μm to about 850 μm. In addition, the steps may include separating the superabsorbent polymer fines having a particle size of less than about 150 μm and recycling said superabsorbent polymer fines to the polymerizing step for forming said superabsorbent polymer particle composition and treating the SAP fines with a caustic solution, and recycling the treated SAP fines in the monomer or gel.

In another embodiment of the present invention, the treated superabsorbent polymer fines can be employed to increase the absorption rate of superabsorbent polymer products when using a carbonate or bicarbonate containing solution to treat the fines, wherein the fines provide a controllable delayed release of the carbonate or bicarbonate agent. It is well known in the art that carbonate or bicarbonate chemicals can act as blowing agents for superabsorbent polymers and create a microcellular expanded hydrogel, which gives fast absorption rates. Treating the superabsorbent polymer fines with such blowing agents provides a controllable delayed release of a gas such as carbon dioxide when dispersed in a monomer solution that is heated or polymerized in accordance with the present invention. The controlled release of the blowing agent in the monomer solution allows control of and improving the microcellular structure of the hydrogel.

The surface cross linked SAP product as used herein is produced by repeating all of the steps for making the SAP particles, up to and including coating the material with a surface crosslinking agent and surface additives, heating said coated polymer, and removing superabsorbent polymer particles greater than about 850 microns and smaller than about 150 microns. The particles smaller than about 300 μm, or about 200 μm, or about 150 μm are treated with a caustic solution and recycled in the polymerization process.

The superabsorbent polymer composition of the present invention exhibits certain characteristics, or properties, as measured by Free Swell Gel Bed Permeability (GBP), Centrifuge Retention Capacity (CRC), absorbency under load at about 0.9 psi (AUL (0.9 psi)), and vortex time. The Free Swell Gel Bed Permeability (GBP) Test is a measurement of the permeability of a swollen bed of superabsorbent material in Darcy (e.g., separate from the absorbent structure) under a confining pressure after what is commonly referred to as “free swell” conditions. In this context, the term “free swell” means that the superabsorbent material is allowed to swell without a swell restraining load upon absorbing test solution as will be described. The Centrifuge Retention Capacity (CRC) Test measures the ability of the superabsorbent polymer composition to retain liquid therein after being saturated and subjected to centrifugation under controlled conditions. The resultant retention capacity is stated as grams of liquid retained per gram weight of the sample (g/g).

The Absorbency Under Load (AUL) Test measures the ability of the superabsorbent polymer composition particles to absorb a 0.9 weight percent solution of sodium chloride in distilled water at room temperature (test solution) while the material is under a load of 0.9 psi. The vortex test measures the amount of time in seconds required for 2 grams of a superabsorbent material to close a vortex created by stirring 50 milliliters of saline solution at 600 revolutions per minute on a magnetic stir plate. The time it takes for the vortex to close is an indication of the free swell absorbing rate of the superabsorbent material.

A superabsorbent polymer particulate composition made by a process of present invention may have a centrifuge retention capacity of from about 5 g/g to about 50 g/g, or from about 10 to about 45 g/g; and an absorption under load at 0.9 psi of from about 10 g/g to about 26 g/g, or from about 13 to about 25 g/g, and a free swell gel bed permeability of from about 1 to about 70 Darcy.

The superabsorbent polymer compositions according to the present invention can be employed in many products including sanitary towels, diapers, or wound coverings, and they have the property that they rapidly absorb large amounts of menstrual blood, urine, or other body fluids. Since the agents according to the invention retain the absorbed liquids even under pressure and are also capable of distributing further liquid within the construction in the swollen state, they are more desirably employed in higher concentrations, with respect to the hydrophilic fiber material, such as fluff, when compared to conventional current superabsorbent compositions. They are also suitable for use as a homogeneous superabsorber layer without fluff content within the diaper construction, as a result of which particularly thin articles are possible. The polymers are furthermore suitable for use in hygiene articles (incontinence products) for adults.

The superabsorbent polymer compositions according to the invention may also be employed in absorbent articles that are suitable for further uses. In particular, the superabsorbent polymer compositions of this invention can be used in absorbent compositions for absorbents for water or aqueous liquids, desirably in constructions for absorption of body fluids, in foamed and non-foamed sheet-like structures, in packaging materials, in constructions for plant growing, as soil improvement agents, or as active compound carriers. For this, they are processed into a web by mixing with paper or fluff or synthetic fibers or by distributing the superabsorbent polymer composition particles between substrates of paper, fluff, or non-woven textiles, or by processing into carrier materials. They are further suited for use in absorbent compositions such as wound dressings, packaging, agricultural absorbents, food trays and pads, and the like.

The present invention may be better understood with reference to the following examples.

Test Procedures

Centrifuge Retention Capacity Test

The Centrifuge Retention Capacity (CRC) Test measures the ability of the superabsorbent polymer to retain liquid therein after being saturated and subjected to centrifugation under controlled conditions. The resultant retention capacity is stated as grams of liquid retained per gram weight of the sample (g/g). The sample to be tested is prepared from particles that are pre-screened through a U.S. standard 30-mesh screen and retained on a U.S. standard 50-mesh screen. As a result, the superabsorbent polymer sample comprises particles sized in the range of about 300 to about 600 microns. The particles can be pre-screened by hand or automatically.

The retention capacity is measured by placing about 0.2 grams of the pre-screened superabsorbent polymer sample into a water-permeable bag that will contain the sample while allowing a test solution (0.9 weight percent sodium chloride in distilled water) to be freely absorbed by the sample. A heat-sealable tea bag material, such as that available from Dexter Corporation (having a place of business in Windsor Locks, Conn., U.S.A.) as model designation 1234T heat sealable filter paper works well for most applications. The bag is formed by folding a 5-inch by 3-inch sample of the bag material in half and heat-sealing two of the open edges to form a 2.5-inch by 3-inch rectangular pouch. The heat seals are about 0.25 inches inside the edge of the material. After the sample is placed in the pouch, the remaining open edge of the pouch is also heat-sealed. Empty bags are also made to serve as controls. Three samples are prepared for each superabsorbent polymer composition to be tested.

The sealed bags are submerged in a pan containing the test solution at about 23° C., making sure that the bags are held down until they are completely wetted. After wetting, the samples remain in the solution for about 30 minutes, at which time they are removed from the solution and temporarily laid on a non-absorbent flat surface.

The wet bags are then placed into the basket wherein the wet bags are separated from each other and are placed at the outer circumferential edge of the basket, wherein the basket is of a suitable centrifuge capable of subjecting the samples to a g-force of about 350. One suitable centrifuge is a CLAY ADAMS DYNAC II, model #0103, having a water collection basket, a digital rpm gauge, and a machined drainage basket adapted to hold and drain the flat bag samples. Where multiple samples are centrifuged, the samples are placed in opposing positions within the centrifuge to balance the basket when spinning The bags (including the wet, empty bags) are centrifuged at about 1,600 rpm (e.g., to achieve a target g-force of about 290 g force with a variance from about 280 to about 300 g force), for 3 minutes. G force is defined as an unit of inertial force on a body that is subjected to rapid acceleration or gravity, equal to 32 ft/sec² at sea level. The bags are removed and weighed, with the empty bags (controls) being weighed first, followed by the bags containing the superabsorbent polymer composition samples. The amount of solution retained by the superabsorbent polymer sample, taking into account the solution retained by the bag itself, is the centrifuge retention capacity (CRC) of the superabsorbent polymer, expressed as grams of fluid per gram of superabsorbent polymer. More particularly, the retention capacity is determined by the following equation:

$\quad\frac{\quad\begin{matrix} {{{sample}\text{/}{bag}\mspace{14mu}{after}{\;\mspace{11mu}}{after}\mspace{14mu}{centrifuge}} -} \\ {{{empty}\mspace{14mu}{bag}\mspace{14mu}{after}\mspace{14mu}{centrifuge}} - {{dry}\mspace{14mu}{sample}\mspace{14mu}{weight}}} \end{matrix}}{\quad{{dry}\mspace{14mu}{sample}\mspace{14mu}{{wei}{ght}}}}$

The three samples are tested, and the results are averaged to determine the Centrifuge Retention Capacity (CRC) of the superabsorbent polymer composition.

Free-Swell Gel Bed Permeability Test (FSGBP)

As used herein, the Free-Swell Gel Bed Permeability Test, also referred to as the Gel Bed Permeability (GBP) Under 0 psi Swell Pressure Test, determines the permeability of a swollen bed of gel particles (e.g., such as the surface treated absorbent material or the superabsorbent material prior to being surface treated), under what is commonly referred to as “free swell” conditions. The term “free swell” means that the gel particles are allowed to swell without a restraining load upon absorbing test solution as will be described. A suitable apparatus for conducting the Gel Bed Permeability Test is shown in FIGS. 1, 2 and 3 and indicated generally as 500. The test apparatus assembly 528 comprises a sample container, generally indicated at 530, and a plunger, generally indicated at 536. The plunger comprises a shaft 538 having a cylinder hole bored down the longitudinal axis and a head 550 positioned at the bottom of the shaft. The shaft hole 562 has a diameter of about 16 mm. The plunger head is attached to the shaft, such as by adhesion. Twelve holes 544 are bored into the radial axis of the shaft, three positioned at every 90 degrees having diameters of about 6.4 mm. The shaft 538 is machined from a LEXAN rod or equivalent material and has an outer diameter of about 2.2 cm and an inner diameter of about 16 mm.

The plunger head 550 has a concentric inner ring of seven holes 560 and an outer ring of 14 holes 554, all holes having a diameter of about 8.8 millimeters as well as a hole of about 16 mm aligned with the shaft. The plunger head 550 is machined from a LEXAN rod or equivalent material and has a height of approximately 16 mm and a diameter sized such that it fits within the cylinder 534 with minimum wall clearance but still slides freely. The total length of the plunger head 550 and shaft 538 is about 8.25 cm, but can be machined at the top of the shaft to obtain the desired mass of the plunger 536. The plunger 536 comprises a 100 mesh stainless steel cloth screen 564 that is biaxially stretched to tautness and attached to the lower end of the plunger 536. The screen is attached to the plunger head 550 using an appropriate solvent that causes the screen to be securely adhered to the plunger head 550. Care must be taken to avoid excess solvent migrating into the open portions of the screen and reducing the open area for liquid flow. Acrylic solvent Weld-on 4 from IPS Corporation (having a place of business in Gardena, Calif., USA) is a suitable solvent.

The sample container 530 comprises a cylinder 534 and a 400 mesh stainless steel cloth screen 566 that is biaxially stretched to tautness and attached to the lower end of the cylinder 534. The screen is attached to the cylinder using an appropriate solvent that causes the screen to be securely adhered to the cylinder. Care must be taken to avoid excess solvent migrating into the open portions of the screen and reducing the open area for liquid flow. Acrylic solvent Weld-on 4 from IPS Corporation (having a place of business in Gardena, Calif., USA) is a suitable solvent. A gel particle sample, indicated as 568 in FIG. 2, is supported on the screen 566 within the cylinder 534 during testing.

The cylinder 534 may be bored from a transparent LEXAN rod or equivalent material, or it may be cut from a LEXAN tubing or equivalent material, and has an inner diameter of about 6 cm (e.g., a cross-sectional area of about 28.27 cm²), a wall thickness of about 0.5 cm and a height of approximately 7.95 cm. A step is machined into the outer diameter of the cylinder 534 such that a region 534 a with an outer diameter of 66 mm exists for the bottom 31 mm of the cylinder 534. An o-ring 540 which fits the diameter of region 534 a may be placed at the top of the step.

The annular weight 548 has a counter-bored hole about 2.2 cm in diameter and 1.3 cm deep so that it slips freely onto the shaft 538. The annular weight also has a thru-bore 548 a of about 16 mm. The annular weight 548 can be made from stainless steel or from other suitable materials resistant to corrosion in the presence of the test solution, which is 0.9 weight percent sodium chloride solution in distilled water. The combined weight of the plunger 536 and annular weight 548 equals approximately 596 grams (g), which corresponds to a pressure applied to the sample 568 of about 0.3 pounds per square inch (psi), or about 20.7 dynes/cm² (2.07 kPa), over a sample area of about 28.27 cm².

When the test solution flows through the test apparatus during testing as described below, the sample container 530 generally rests on a weir 600. The purpose of the weir is to divert liquid that overflows the top of the sample container 530 and diverts the overflow liquid to a separate collection device 601. The weir can be positioned above a scale 602 with a beaker 603 resting on it to collect saline solution passing through the swollen sample 568.

To conduct the Gel Bed Permeability Test under “free swell” conditions, the plunger 536, with the weight 548 seated thereon, is placed in an empty sample container 530 and the height from the top of the weight 548 to the bottom of the sample container 530 is measured using a suitable gauge accurate to 0.01 mm. The force the thickness gauge applies during measurement should be as low as possible, preferably less than about 0.74 Newtons. It is important to measure the height of each empty sample container 530, plunger 536, and weight 548 combination and to keep track of which plunger 536 and weight 548 is used when using multiple test apparatus. The same plunger 536 and weight 548 should be used for measurement when the sample 568 is later swollen following saturation. It is also desirable that the base that the sample cup 530 is resting on is level, and the top surface of the weight 548 is parallel to the bottom surface of the sample cup 530.

The sample to be tested is prepared from superabsorbent polymer composition particles which are prescreened through a U.S. standard 30 mesh screen and retained on a U.S. standard 50 mesh screen. As a result, the test sample comprises particles sized in the range of about 300 to about 600 microns. The superabsorbent polymer particles can be pre-screened with, for example, a RO-TAP Mechanical Sieve Shaker Model B available from W. S. Tyler, Inc., Mentor, Ohio. Sieving is conducted for 10 minutes. Approximately 2.0 grams of the sample is placed in the sample container 530 and spread out evenly on the bottom of the sample container. The container, with 2.0 grams of sample in it, without the plunger 536 and weight 548 therein, is then submerged in the 0.9% saline solution for a time period of about 60 minutes to saturate the sample and allow the sample to swell free of any restraining load. During saturation, the sample cup 530 is set on a mesh located in the liquid reservoir so that the sample cup 530 is raised slightly above the bottom of the liquid reservoir. The mesh does not inhibit the flow of saline solution into the sample cup 530. A suitable mesh can be obtained as part number 7308 from Eagle Supply and Plastic, having a place of business in Appleton, Wis., U.S.A. Saline does not fully cover the superabsorbent polymer composition particles, as would be evidenced by a perfectly flat saline surface in the test cell. Also, saline depth is not allowed to fall so low that the surface within the cell is defined solely by swollen superabsorbent, rather than saline.

At the end of this period, the plunger 536 and weight 548 assembly are placed on the saturated sample 568 in the sample container 530 and then the sample container 530, plunger 536, weight 548, and sample 568 are removed from the solution. After removal and before being measured, the sample container 530, plunger 536, weight 548, and sample 568 are to remain at rest for about 30 seconds on a suitable flat, large grid non-deformable plate of uniform thickness. The thickness of the saturated sample 568 is determined by again measuring the height from the top of the weight 548 to the bottom of the sample container 530, using the same thickness gauge used previously provided that the zero point is unchanged from the initial height measurement. The sample container 530, plunger 536, weight 548, and sample 568 may be placed on a flat, large grid non-deformable plate of uniform thickness that will prevent liquid in the sample container from being released onto a flat surface due to surface tension. The plate has an overall dimension of 7.6 cm by 7.6 cm, and each grid has a cell size dimension of 1.59 cm long by 1.59 cm wide by 1.12 cm deep. A suitable flat, large grid non-deformable plate material is a parabolic diffuser panel, catalogue number 1624K27, available from McMaster Carr Supply Company, having a place of business in Chicago, Ill., U.S.A., which can then be cut to the proper dimensions. This flat, large mesh non-deformable plate must also be present when measuring the height of the initial empty assembly. The height measurement should be made as soon as practicable after the thickness gauge is engaged. The height measurement obtained from measuring the empty sample container 530, plunger 536, and weight 548 is subtracted from the height measurement obtained after saturating the sample 568. The resulting value is the thickness, or height “H” of the swollen sample.

The permeability measurement is initiated by delivering a flow of the 0.9% saline solution into the sample container 530 with the saturated sample 568, plunger 536, and weight 548 inside. The flow rate of test solution into the container is adjusted to cause saline solution to overflow the top of the cylinder 534 thereby resulting in a consistent head pressure equal to the height of the sample container 530. The test solution may be added by any suitable means that is sufficient to ensure a small, but consistent amount of overflow from the top of the cylinder, such as with a metering pump 604. The overflow liquid is diverted into a separate collection device 601. The quantity of solution passing through the sample 568 versus time is measured gravimetrically using the scale 602 and beaker 603. Data points from the scale 602 are collected every second for at least sixty seconds once the overflow has begun. Data collection may be taken manually or with data collection software. The flow rate, Q, through the swollen sample 568 is determined in units of grams/second (g/s) by a linear least-square fit of fluid passing through the sample 568 (in grams) versus time (in seconds).

Permeability in cm² is obtained by the following equation: K=[Q*H*μ]/[A*ρ*P], where K=Permeability (cm²), Q=flow rate (g/sec), H=height of swollen sample (cm), μ=liquid viscosity (poise) (approximately one centipoise for the test solution used with this Test), A=cross-sectional area for liquid flow (28.27 cm² for the sample container used with this Test), ρ=liquid density (g/cm³) (approximately one g/cm³, for the test solution used with this Test) and P=hydrostatic pressure (dynes/cm²) (normally approximately 7,797 dynes/cm²). The hydrostatic pressure is calculated from P=ρ*g*h, where ρ=liquid density (g/cm³), g=gravitational acceleration, nominally 981 cm/sec², and h=fluid height, e.g., 7.95 cm for the Gel Bed Permeability Test described herein.

A minimum of two samples is tested and the results are averaged to determine the gel bed permeability of the sample.

Absorbency Under Load Test (AUL0.9 psi)

The Absorbency Under Load (AUL) Test measures the ability of the superabsorbent polymer composition particles to absorb a 0.9 weight percent solution of sodium chloride in distilled water at room temperature (test solution) while the material is under a load of 0.9 psi. The apparatus for testing AUL consists of:

-   -   An AUL assembly including a cylinder, a 4.4 g piston, and a         standard 317 gm weight. The components of this assembly are         described in additional detail below.     -   A flat-bottomed square plastic tray that is sufficiently broad         to allow the glass fits to lay on the bottom without contact         with the tray walls. A plastic tray that is 9″ by 9″(22.9         cm×22.9 cm), with a depth of 0.5 to 1″(1.3 cm to 2.5 cm) is         commonly used for this test method.     -   A 12.5 cm diameter sintered glass frit with a ‘C’ porosity         (25-50 microns). This frit is prepared in advance through         equilibration in saline (0.9% sodium chloride in distilled         water, by weight). In addition to being washed with at least two         portions of fresh saline, the frit must be immersed in saline         for at least 12 hours prior to AUL measurements.     -   Whatman Grade 1, 12.5 cm diameter filter paper circles.     -   A supply of saline (0.9% sodium chloride in distilled water, by         weight).

Referring to FIG. 4, the cylinder 412 of the AUL assembly 400 used to contain the superabsorbent polymer composition particles 410 is made from one-inch (2.54 cm) inside diameter thermoplastic tubing machined-out slightly to be sure of concentricity. After machining, a 400 mesh stainless steel wire cloth 414 is attached to the bottom of the cylinder 412 by heating the steel wire cloth 414 in a flame until red hot, after which the cylinder 412 is held onto the steel wire cloth until cooled. A soldering iron can be utilized to touch up the seal if unsuccessful or if it breaks. Care must be taken to maintain a flat smooth bottom and not distort the inside of the cylinder 412.

The 4.4 g piston (416) is made from one-inch diameter solid material (e.g., PLEXIGLAS®) and is machined to closely fit without binding in the cylinder 412.

A standard 317 gm weight 418 is used to provide a 62,053 dyne/cm² (about 0.9 psi) restraining load. The weight is a cylindrical, 1 inch (2.5 cm) diameter, stainless steel weight that is machined to closely fit without binding in the cylinder.

Unless specified otherwise, a sample 410 corresponding to a layer of at least about 300 gsm. (0.16 g) of superabsorbent polymer composition particles is utilized for testing the AUL. The sample 410 is taken from superabsorbent polymer composition particles that are pre-screened through U.S. standard #30 mesh and retained on U.S. std. #50 mesh. The superabsorbent polymer composition particles can be pre-screened with, for example, a RO-TAP® Mechanical Sieve Shaker Model B available from W. S. Tyler, Inc., Mentor, Ohio. Sieving is conducted for about 10 minutes.

The inside of the cylinder 412 is wiped with an antistatic cloth prior to placing the superabsorbent polymer composition particles 410 into the cylinder 412.

The desired amount of the sample of sieved superabsorbent polymer composition particles 410 (about 0.16 g) is weighed out on a weigh paper and evenly distributed on the wire cloth 414 at the bottom of the cylinder 412. The weight of the superabsorbent polymer composition particles in the bottom of the cylinder is recorded as ‘SA,’ for use in the AUL calculation described below. Care is taken to be sure no superabsorbent polymer particles cling to the wall of the cylinder. After carefully placing the 4.4 g piston 412 and 317 g weight 418 on the superabsorbent polymer composition particles 410 in the cylinder 412, the AUL assembly 400 including the cylinder, piston, weight, and superabsorbent polymer composition particles is weighed, and the weight is recorded as weight ‘A’.

A sintered glass frit 424 (described above) is placed in the plastic tray 420, with saline 422 added to a level equal to that of the upper surface of the glass frit 424. A single circle of filter paper 426 is placed gently on the glass frit 424, and the AUL assembly 400 with the superabsorbent polymer composition particles 410 is then placed on top of the filter paper 426. The AUL assembly 400 is then allowed to remain on top of the filter paper 426 for a test period of one hour, with attention paid to keeping the saline level in the tray constant. At the end of the one hour test period, the AUL apparatus is then weighed, with this value recorded as weight ‘B.’

The AUL (0.9 psi) is calculated as follows: AUL(0.9 psi)=(B−A)/SA

wherein

A=Weight of AUL Unit with dry SAP

B=Weight of AUL Unit with SAP after 60 minutes absorption

SA=Actual SAP weight

A minimum of two tests is performed and the results are averaged to determine the AUL value under 0.9 psi load. The samples are tested at about 23° C. and about 50% relative humidity.

Vortex Time Test

The vortex test measures the amount of time in seconds required for 2 grams of a superabsorbent material to close a vortex created by stirring 50 milliliters of saline solution at 600 revolutions per minute on a magnetic stir plate. The time it takes for the vortex to close is an indication of the free swell absorbing rate of the superabsorbent material.

Equipment and Materials

1. Schott Duran 100 ml Beaker and 50 ml graduated cylinder.

2. Programmable magnetic stir plate, capable of providing 600 revolutions per minute (such as that commercially available from PMC Industries, under the trade designation Dataplate® Model #721).

3. Magnetic stir bar without rings, 7.9 millimeters.times.32 millimeters, Teflon® covered (such as that commercially available from Baxter Diagnostics, under the trade designation S/PRIM. brand single pack round stirring bars with removable pivot ring).

4. Stopwatch

5. Balance, accurate to +/−0.01 g

6. Saline solution, 0.87 w/w % Blood Bank Saline available from Baxter Diagnostics (considered, for the purposes of this application to be the equivalent of 0.9 wt. % saline

7. Weighing paper

8. Room with standard condition atmosphere: Temp=23° C.+/−1° C. and Relative Humidity=50%+/−2%.

Test Procedure

1. Measure 50 ml+/−0.01 ml of saline solution into the 100 ml beaker.

2. Place the magnetic stir bar into the beaker.

3. Program the magnetic stir plate to 600 revolutions per minute.

4. Place the beaker on the center of the magnetic stir plate such that the magnetic stir bar is activated. The bottom of the vortex should be near the top of the stir bar.

5. Weigh out 2 g+/−0.01 g of the superabsorbent material to be tested on weighing paper.

NOTE: The superabsorbent material is tested as received (i.e. as it would go into an absorbent composite such as those described herein). No screening to a specific particle size is done, though the particle size is known to have an effect on this test.

6. While the saline solution is being stirred, quickly pour the superabsorbent material to be tested into the saline solution and start the stopwatch. The superabsorbent material to be tested should be added to the saline solution between the center of the vortex and the side of the beaker. 7. Stop the stopwatch when the surface of the saline solution becomes flat and record the time. 8. The time, recorded in seconds, is reported as the Vortex Time.

EXAMPLES

The following examples for the examples are provided to illustrate the invention and do not limit the scope of the claims. Unless otherwise stated all parts, and percentages are by weight.

General Procedure for Preparing Superabsorbent Polymer Material and Fines

It is generally understood that superabsorbent polymer fines may include particles having a particle size of about 150 μm or less, which may be recovering from the grinding process in making SAP particles. The following example provides information about making the SAP and fines.

Into a polyethylene vessel equipped with an agitator and cooling coils was added, 2.0 kg of 50% NaOH to 3.32 kg of distilled water and cooled to 20° C. 0.8 kg of glacial acrylic acid was then added to the caustic solution and the solution again cooled to 20° C. 4.32 g of polyethylene glycol monoallylether acrylate, 4.32 g of ethoxylated trimethylol propane triacrylate SARTOMER® 454 product, and 1.6 kg of glacial acrylic acid were added to the first solution, followed by cooling to 4-6° C. Fines were added into the monomer solution with agitation. Nitrogen was bubbled through the monomer solution for about 5 minutes. The monomer solution was then discharged into a rectangular tray. 80 g of 1% by weight of H₂O₂ aqueous solution, 120 g of 2 wt % aqueous sodium persulfate solution, and 72 g of 0.5 wt % aqueous sodium erythorbate solution was added into the monomer solution to initiate polymerization reaction. The agitator was stopped and the initiated monomer was allowed to polymerize for 20 minutes. The resulting hydrogel was chopped and extruded with a Hobart 4M6 commercial extruder, followed by drying in a Procter & Schwartz Model 062 forced air oven at 175° C. for 12 minutes with up flow and 6 minutes with down flow air on a 20 in X 40 in perforated metal tray to a final product moisture level of less than 5 wt %. The dried material was coarse-ground in a Prodeva Model 315-S crusher, milled in an MPI 666-F three-stage roller mill and sieved with a Minox MTS 600DS3V to remove particles greater than 850 μm and smaller than 150 μm. The particles that are smaller than 150 microns are SAP fines.

The particles between 150 μm and 850 μm were coated with 1 wt % of ethylene carbonate, 3 wt % of water using a 25 wt % aqueous solution, and surface additives if applicable. The coated sample was then heated in a convection oven at 185° C. for surface crosslinking The surface crosslinked particulate material was then sieved with 20/100 mesh US standard sieves to remove particles greater than 850 μm and smaller than 150 μm. The particles that are smaller than 150 microns are surface crosslinked fines.

General Procedure for Treating Fines with Caustic Solutions

400 g of SAP fines were stirred in a Kitchen-Aid mixer. A Paasche airbrush sprayer was used to apply the caustic solutions, with the mixer on high speed while liquid was spraying. The treated fines were allowed to relax for at least 5 minutes prior to reworking into polymerization batches.

Examples 1, 2 and Comparative Example A

Surface crosslinked fines were treated with NaOH using a 50 wt % aqueous solution. The treated fines were added into the monomer solution in which the content of the superabsorbent polymer fines relative to the total amount of the monomer solution is 1.9 wt % (5 wt % fines add-on based on the dry weight of fines and the preproduct). In comparative example A, the untreated fines were added directly into the monomer solution. The results are summarized in Table 1 below. Compared with the preproduct prepared with untreated fines, the products with treated fines demonstrated higher CRC.

TABLE 1 NaOH add-on PSXM Example Sample level (%) CRC (g/g) Comparative 233-21-2 0 38.6 Example A Example 1 233-8-4 6 41.8 Example 2 233-8-5 9 42.0 NaOH treatment level was based on the weight of NaOH solid and fines

Examples 3, 4 and Comparative Example B

The preproducts listed in Table 1 were coated with 1 wt % of ethylene carbonate and 3 wt % of water, respectively. The coated sample was then heated in a convection oven at 185° C. for surface crosslinking The results are summarized in Table 2 below. Compared with the SXL product prepared with untreated fines, the products with treated fines demonstrated higher 0.9 psi AUL.

TABLE 2 NaOH SXL 0.9 psi treatment Time CRC AUL Example level (%) (min) (g/g) (g/g) Comparative 0 30 33.3 17.9 Example B Example 3 6 40 34.6 21.1 Example 4 9 50 34 23.3 NaOH treatment level was based on the weight of NaOH solid and fines

Examples 5-8 and Comparative Examples C and D

The preproducts listed in Table 1 were coated with 0.5 wt % of Sipernat® 22s, 1 wt % of ethylene carbonate, and 3 wt % of water, respectively. The coated sample was then heated in a convection oven at 185° C. for surface crosslinking The results are summarized in Table 3 below. Compared with the SXL products containing untreated fines, the products with treated fines demonstrated higher 0.9 psi AUL and higher GBP.

TABLE 3 NaOH SXM 0.9 treatment Time CRC AUL GBP Example level (%) (min) (g/g) (g/g) (Darcy) Comparative 0 30 32.9 12.9 28.8 Example C Comparative 0 40 31.9 14.1 38.4 Example D Example 5 6 40 32.8 15.3 47.7 Example 6 6 50 31.8 16.1 59.4 Example 7 9 40 33.2 14.7 41.9 Example 8 9 50 32.8 16.4 48.2 NaOH treatment level was based on the weight of NaOH solid and fines

Examples 9 and 10

Preproduct fines were treated with Na₂CO₃ using a 25 wt % aqueous solution. The treated fines were added into the monomer solution in which the content of the superabsorbent polymer fines relative to the total amount of the monomer solution is 5.7 (15 wt % fines add-on based on the dry weight of fines and preproduct). The results are summarized in Table 4 below. The products prepared with Na₂CO₃ treated fines afforded fast absorption speed. Products prepared with untreated fines generally show the vortex time higher than 40 seconds.

TABLE 4 Na₂CO₃ treatment PSXM CRC Example level (%) (g/g) Vortex time (sec) Example 9 2.5 37.8 32 Example 10 5.0 34.2 22 Na₂CO₃ treatment level was based on the weight of Na₂CO₃ solid and fines

Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding approaches.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements. 

1. A process for recycling superabsorbent polymer fines in the production of superabsorbent polymer gel, compromising the steps of: a) dissolving at least one acrylic acid monomer and/or its sodium salt and a cross-linking agent in an aqueous solution to form a monomer solution; b) polymerizing the mixture obtained in step a) to produce a superabsorbent polymer gel; c) treating the superabsorbent polymer fines with a caustic solution containing from about 0.1 to about 12% caustic based on the weight of the superabsorbent polymer fines to form caustic treated superabsorbent polymer fines; and d) mixing the caustic treated superabsorbent polymer fines obtained in step c) with the superabsorbent polymer gel obtained in step b) to provide an aqueous fluid absorbent polymer, wherein the content of the superabsorbent polymer fines relative to the total amount of the superabsorbent polymer gel is from about 0.1 to about 30 wt %.
 2. The process of claim 1 further comprising the steps to make a superabsorbent polymer particulate wherein the steps comprise comminuting the aqueous fluid absorbent polymer from said polymerizing and mixing step; drying said comminuted polymer; and separating said dried superabsorbent polymer particulate into a portion having a desired particle size from about 150 μm to about 850 μm as measured by screening through a U.S. standard 20 mesh screen and retained on a U.S. standard 100 mesh screen.
 3. The process of claim 2 further comprising the steps to make a superabsorbent polymer composition wherein the steps comprise coating the superabsorbent polymer particulate with a surface crosslinking agent and surface additives; heating said coated polymer; and separating said dried superabsorbent polymer particulate into a portion having a desired particle size from about 150 μm to about 850 μm as measured by screening through a U.S. standard 20 mesh screen and retained on a U.S. standard 100 mesh screen.
 4. The process of claim 1, further comprising the steps of separating the superabsorbent polymer fines having a particle size of less than about 150 μm; treating the superabsorbent polymer fines having a particle size of less than about 150 μm with a caustic solution containing from about 0.1 to about 12% caustic based on the weight of the superabsorbent polymer fines; and recycling said treated superabsorbent polymer fines having a particle size of less than about 150 μm to the polymerizing step b) for forming said superabsorbent polymer gel.
 5. The process of claim 3 further comprising the steps of separating the superabsorbent polymer fines having a particle size of less than about 150 μm; treating the superabsorbent polymer fines having a particle size of less than about 150 μm with a caustic solution containing from about 0.1 to about 12% caustic based on the weight of the superabsorbent polymer fines; and recycling said treated superabsorbent polymer fines having a particle size of less than about 150 μm to the polymerizing step b) for forming said superabsorbent polymer gel.
 6. The process of claim 1 wherein the caustic is selected from sodium carbonate or sodium bicarbonate.
 7. The process of claim 1 wherein said polymer gel is a cross linked polymer of polyacrylic acid, sodium polyacrylate or copolymers thereof.
 8. A superabsorbent polymer particulate composition made by the process of claim 3 wherein at least about 40% by weight of the superabsorbent polymer particulate has a particle size from about 300 μm to about 600 μm as measured by screening through a U.S. standard 30 mesh screen and retained on a U.S. standard 50 mesh screen.
 9. A superabsorbent polymer particulate composition made by the process of claim 2 having a free swell gel bed permeability of from about 1 to about 70 Darcy.
 10. An absorbent composition comprising the superabsorbent polymer particle composition made by the process of claim
 3. 11. A sanitary article comprising the superabsorbent polymer particle composition made by the process of claim
 3. 